Wireless communication systems that provide communication services to wireless communication devices (often denoted by UE, which is short for user equipment) such as mobile phones, smartphones etc., have evolved during the last decade into systems that must meet an ever increasing demand for high speed data communication capabilities in terms of, e.g., bitrate and to provide these capabilities at any given time and at any geographical location.
The evolution of the systems has followed a sequence of “generations”, from first generation analog systems and second generation (2G) digital systems that were mainly focused on providing circuit switched voice services, via third generation (3G) systems that were capable of also providing moderately high data rate packet switched services, to the current fourth generation (4G) systems in which all services are provided in terms of packet data services. A widespread 4G standard is the third generation partnership project (3GPP) long term evolution (LTE), according to which information is communicated in the form of stream of symbols encoded by amplitude and phase of radio frequency (RF) signals distributed over a plurality of sub-carrier frequencies, i.e. according to an orthogonal frequency division multiplex (OFDM) standard.
The work of defining a fifth generation (5G) wireless communication standard is very comprehensive and a future 5G standard should support a variety of different use cases such as mobile broadband (MBB) with massive multiple input-multiple output (MIMO) radio link support, low latency, high reliability communication, low cost machine type communication (MTC) as well as supporting frequency bands spanning from sub GHz to 50+ GHz. In order to be able to support such use cases, the current work within 3GPP includes proposals to enable—within a system bandwidth—a mix of different OFDM sub-carrier spacings (i.e. having different OFDM symbol lengths, and thereby different sampling rates). For example, sub-carrier spacing of 15 kHz*2n, for a limited set of n will be supported, such as 15 kHz and 60 kHz. Furthermore, in order to optimize communication, it is also on the agenda to separate the control plane (for instance mobility management) from the data plane, in the sense that some of the control data may be transmitted from a first network (NW) node (for instance having wide area coverage) while the data may be transmitted from another NW node (for instance, indoor hotspot coverage). Furthermore, it may be possible that the control plane, supporting large (macro) cells may have one sub-carrier spacing (e.g. 15 kHz) and, while data (for low latency applications) may be transmitted with a larger sub-carrier spacing (e.g. 60 kHz), and thereby shorter OFDM symbols enabling lower latency.
Since a carrier frequency generated in a wireless communication device is generated by a low cost local oscillator, the carrier frequency (and thereby also the timing) may drift in relation to the (typically more stable) NW node with which the wireless communication device communicates. Therefore, the wireless communication device needs to continuously monitor frequency and/or timing (f/t) synchronization with respect to the serving NW node. Furthermore, in case of mixed numerologies of the sub-carrier spacing, a wireless communication device supporting 5G needs to keep track (in terms of f/t-synchronization) of all different sub-carrier spacing of signals with possible data intended for the wireless communication device. That is, referring to the 5G example above, the wireless communication device needs to keep track of sub-carrier spacing 15 kHz for the mobility management information and 60 kHz sub-carrier spacing for the data packets.
Furthermore, in a more general aspect, the NW nodes transmitting data to the wireless communication device may be transparent from the point of view of the wireless communication device, and hence the wireless communication device does not necessarily know whether the 15 kHz and 60 kHz numerologies are transmitted from the same NW node or not. Applying prior art techniques for fit synchronization means that the wireless communication device always has two independent synchronization procedures, one for respective numerology. This may entail unnecessary high power consumption.